Nuclear transfer experiments in oocytes can provide valuable information on differentation of embryonic cell lines during development. Due to the size of mammalian oocytes and their sensitivity to in vitro conditions and injuries inflicted during micromanipulation, success in this area has been minimal in mammals. To this end, we are developing new techniques to minimize embryo loss due to damage to the oolemma. These techniques under development are 1) to extrude the meiotic spindle into the perivitelline space, which is subsequently removed with a micromanipulator, and 2) to introduce the new nucleus utilizing membrane fusion techniques. The enucleation procedure without penetrating the ooplasm with micropipettes is presently under development utilizing a combination of proper regimen of microtubule and microfilament inhibitors. To test whether a new nucleus can be introduced into the ooplasts we demonstrated that polyethylene glycol can be utilized for this purpose. The results indicate that it is feasible to use membrane fusion techniques with oocytes. To further demonstrate this point, unfertilized eggs were fused. To date diploid oocyte fusion products can progress through cleavage and upon transfer to foster mothers, embryos developed to the 14 somite stage. The new techniques will be utilized to explore the developmental restriction of nuclei of embryonic cells as well as the totipotency of teratoma or other cell lines.